Organic light emitting diodes (OLEDs) are among the research focuses in the display field. As compared with liquid crystal displays (LCDs), OLEDs have advantages of low energy consumption, low production cost, self-light emitting, wide viewing angles, and fast response. Currently, OLED display screens start to replace conventional LCD display screens in fields like mobile phones, PDAs, digital cameras. Design of a pixel circuit is one of the core techniques of the OLED display device, and has important research significance.
The brightness of LCDs is controlled by a stable voltage, while OLEDs are driven by a current, and a stable current is required to control light emitting. Due to process factors and device aging, driving transistors in the pixel circuit are non-uniform in the threshold voltage Vth. As a result, different currents flow through different pixels in the OLED, so that these pixels are not uniform in brightness, which affects the display effect of the whole image.
An existing 2T1C pixel circuit is shown in FIG. 1. The pixel circuit consists of one driving transistor T2, one switch transistor T1 and one storage capacitor Cs. When a scan line SL selects a row, the scan line SL inputs a low level signal, so that the P-type switch transistor T1 turns on, and a voltage of a data line DL is written into the storage capacitor Cs. After the row is scanned, the signal input by scan line SL changes to a high level, the P-type switch transistor T1 is cut off, and the voltage stored in the storage capacitor Cs controls the driving transistor T2 to produce a current for driving OLED pixels, which ensures the OLED pixels emit light continuously in a frame. The driving transistor T2 has a saturation current IOLED=K(VSG−Vth)2, wherein VSG is a voltage difference between a source and a gate of the driving transistor T2, K is a structural parameter, and Vth is a threshold voltage of the driving transistor T2. As mentioned above, the threshold voltage Vth of the driving transistor T2 may drift due to process factors and device aging. As a result, currents flowing through different pixels in the OLED vary with the variation of the threshold voltage Vth of the driving transistor, which leads to non-uniform image brightness.